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参数资料
| 型号： | MIC28500 2A EV |
| 厂商： | Micrel Inc |
| 文件页数： | 15/29页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR MIC28500 2A |
| 标准包装： | 1 |
| 系列： | Hyper Speed Control™，SuperSwitcher II™ |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，非隔离 |
| 输出电压： | 12V |
| 电流 - 输出： | 2A |
| 输入电压： | 30 ~ 75V |
| 稳压器拓扑结构： | 降压 |
| 频率 - 开关： | 250kHz |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | MIC28500 |
| 其它名称： | 576-4019 |
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Micrel, Inc.
Figure 3 shows the operation of the MIC28500 during a
load transient. The output voltage drops due to the
sudden load increase, which causes the V FB to be less
than V REF . This will cause the error comparator to trigger
an ON-time period. At the end of the ON-time period, a
minimum OFF-time t OFF(min) is generated to charge C BST
since the feedback voltage is still below V REF . Then, the
next ON-time period is triggered due to the low feedback
voltage. Therefore, the switching frequency changes
during the load transient, but returns to the nominal fixed
frequency once the output has stabilized at the new load
current level. With the varying duty cycle and switching
frequency, the output recovery time is fast and the
output voltage deviation is small in MIC28500 converter.
Figure 3. MIC28500 Load Transient Response
Unlike true current-mode control, the MIC28500 uses the
output voltage ripple to trigger an ON-time period. The
MIC28500
Soft-Start
Soft-start reduces the power supply input surge current
at startup by controlling the output voltage rise time. The
input surge appears while the output capacitor is
charged up. A slower output rise time will draw a lower
input surge current.
The MIC28500 implements an internal digital soft-start
by making the 0.8V reference voltage V REF ramp from 0
to 100% in about 6ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case V FB ramp. Once the soft-start cycle ends, the
related circuitry is disabled to reduce current
consumption. V DD must be powered up at the same time
or after V IN to make the soft-start function correctly.
Current Limit
The MIC28500 uses the R DS(ON) of the internal low-side
power MOSFET to sense over-current conditions. This
method will avoid adding cost, board space and power
losses taken by a discrete current sense resistor. The
low-side MOSFET is used because it displays much
lower parasitic oscillations during switching than the
high-side MOSFET.
In each switching cycle of the MIC28500 converter, the
inductor current is sensed by monitoring the low-side
MOSFET in the OFF period. If the peak inductor current
is greater than 7A, then the MIC28500 turns off the high-
side MOSFET and a soft-start sequence is triggered.
This mode of operation is called “hiccup mode” and its
purpose is to protect the downstream load in case of a
hard short. The current-limit threshold has a foldback
characteristic related to the feedback voltage, as shown
in Figure 4.
output voltage ripple is proportional to the inductor
current ripple if the ESR of the output capacitor is large
enough.
In order to meet the stability requirements, the
MIC28500 feedback voltage ripple should be in phase
with the inductor current ripple and large enough to be
sensed by the g m amplifier and the error comparator.
The recommended feedback voltage ripple is
20mV~100mV. If a low-ESR output capacitor is selected,
then the feedback voltage ripple may be too small to be
sensed by the g m amplifier and the error comparator.
Also, the output voltage ripple and the feedback voltage
ripple are not necessarily in phase with the inductor
current ripple if the ESR of the output capacitor is very
12
10
8
6
4
2
0
Current Limit Threshold
vs. Feedback Voltage
V IN = 48V
low. In these cases, ripple injection is required to ensure
0.0
0.2
0.4
0.6
0.8
1.0
proper operation. Please refer to “Ripple Injection”
subsection in Application Information for more details
about the ripple injection technique.
FEEDBACK VOLTAGE (V)
Figure 4. MIC28500 Current Limit Foldback Characteristic
June 2011
15
M9999-060311-B
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